1. Field of the Invention
The present invention relates to polymer supported transition metal catalysts and more particularly to the preparation of these catalysts by photoirradiation of the polymer support in the presence of a transition metal compound and to the use of the catalysts in chemical reactions under mild conditions.
2. Description of the Prior Art
The abundant reserves of coal, coupled with the rapid depletion of known oil reserves, mandates that new technology be developed for synthesis of synthetic fuels from coal gases. Hydroformylation, or the oxo reaction of an unsaturated olefinic compound with hydrogen and carbon monoxide to yield an aldehyde, has been known since 1938. This reaction is interesting since a second stage process using hydrogen can convert the aldehyde into the so-called oxo alcohols.
The catalytic mechanism of the oxo process is homogenous in nature and the process was therefore developed using homogenous catalysts such as dicobalt octacarbonyl, Co.sub.2 (CO).sub.8 or analogues thereof utilizing other low valent transition metals or replacing one or more of the carbonyl ligands with other ligands such as triphenylphosphine. The process is usually practiced at high temperatures and pressures. Examples of current industrial processes for forming products from coal based raw materials follows: ##STR1##
The relative merits of homogeneous and heterogeneous catalysts are well known. Homogeneous catalysts have better defined active sites, usually have all of the metal available for catalysis, and offer steric and electronic environments of the metal atom that can, at least in principle, be varied at will. The major disadvantage of homogeneous catalysts is the need to separate them from reaction products without loss of their valuable metal content. This step can be both complex and expensive. Other disadvantages are that these catalysts are relatively easily deactivated through aggregation or by poisonous by-products or at extreme temperatures. Also corrosion of reactors by metal complexes is common.
The advantages might be retained and the disadvantages removed if the homogeneous catalyst is either impregnated onto a solid support, or in some way chemically bonded to it.
Most workers have used complexes in which phosphine groups are used to link the metal to the solid support. Two types of polymer support, modified polystyrene and silica, have been studied. With polystyrene the form of the polymer can be changed by changing the amount of cross-linking, a feature that appears to have important consequences on the type of catalyst produced. This is the type of support that has been most widely used. Complexes of cobalt carbonyls with polyvinylpyridine have been utilized to thermally catalyze hydroformylation at high temperature and pressure. It has been found that addition of polymer made the system resistant to poisons. However, the catalyst has been shown to be a solution of cobalt hydrocarbonyl and is dissociated from the polymer support. Thus, a disadvantage of thermal hydroformylation using metal polymer species is the gradual loss of metal from the polymer, rendering it commercially unfeasible to utilize expensive metals such as rhodium as a catalyst component.
Hydroformylation under photochemical conditions in solution has also been observed. The overall stoichiometry utilizing Co.sub.2 (CO).sub.8 appears to be as follows: EQU 2Co.sub.2 (CO).sub.8 + 4RCH = CH.sub.2 + 4H.sub.2 .sup.h.nu. Co.sub.4 (CO).sub.12 + 4RCH.sub.2 CH.sub.2 CHO/RCH(CHO)CH.sub.3
as in thermal catalytic systems, the reaction is inhibited by carbon monoxide. The cobalt dimer is converted in part to the tetramer and is consumed by the process which is therefore not catalytic in the homogeneous phase. The carbonyl group being added to the olefin therefore originates from the cobalt dimer. The tetramer absorbs in the visible at longer wavelengths than the dimer as well as in the same region with higher intensity. Consequently the photoreaction is self-terminating due to the internal filter effect.
Transition metal-phosphinated polystyrene complexes have also been prepared in the presence of light. However, these catalysts though useful in hydrogenation reactions, have not demonstrated hydroformylation activity.